Passive aerosol diluter mechanism

ABSTRACT

Various embodiments include methods and systems to dilute a sampled particle-laden aerosol stream. In one embodiment, a system to dilute a sampled aerosol stream includes an aerosol sample inlet. A filter is coupled in fluid communication with and in parallel with a flow-monitoring device to receive the sampled aerosol stream from the aerosol sample inlet. The flow-monitoring device is configured to allow for a passage of particles contained in the sampled aerosol stream. A pressure sensor and a temperature sensor monitor the filter and the flow-monitoring device. An output from the filter and the flow-monitoring device may be directed to particle measurement or particle sizing instrumentation. An actual dilution ratio of the output sent to the particle measurement or particle sizing instrumentation is determined based on a nominal flowrate of the flow-monitoring device and thermodynamic properties of a gas comprising the aerosol stream. Other methods and apparatuses are disclosed.

PRIORITY APPLICATION

This application claims the benefit of priority to U.S. ProvisionalApplication Ser. No. 62/394,723, filed on Sep. 14, 2016, which isincorporated herein by reference in its entirety.

BACKGROUND

In a number of airborne particle-measurement and particle-concentrationstudies, a condensation particle counter (CPC, also known as acondensation nucleus counter (CNC)) is used to detect particles in amonitored environment. In a CPC, particles can be detected that are toosmall to scatter enough light to be detected by conventional detectiontechniques (e.g., light scattering of a laser beam in an opticalparticle counter, OPC). The small particles are grown to a larger sizeby condensation formed on the particle. That is, each particle serves asa nucleation point for the working fluid; a vapor, which is produced bythe instrument's working fluid, is condensed onto the particles to makethe particles larger. After achieving growth of the particle due tocondensation of the working fluid vapor onto the particle, CPCs functionsimilarly to optical particle counters in that the individual dropletsthen pass through the focal point (or line) of a laser beam, producing aflash of light in the form of scattered light. Each light flash iscounted as one particle.

However, in certain environments, such as air pollution measurements,engine exhaust research, and regulatory studies involving measurement ofsize or concentrations of particles in an aerosol stream, theconcentration of particles is too high to measure accurately with a CPC.Often, such particle-measurement methods and procedures are defined by agovernmental agency, such as the United States Environmental ProtectionAgency (EPA) or the California Air Resources Board (CARB). Often, aconcentration of particles is too high to measure accurately with a CPC.In these environments, particle concentrations can range up to 5×10⁹particles per cubic centimeter or higher. However, many CPCs can onlymeasure particle concentrations accurately at much lower concentrationranges (e.g., perhaps a factor of 10⁴ lower than the stated particleconcentration above) before inaccurate monitoring occurs due tocoincidence errors (counting two or more particles in an aerosol samplestream simultaneously). Although various capillary-type and orificedilutors are known in the art, none are capable of accurately providinga known dilution ratio under circumstances of varying temperature andabsolute pressure. The problem is often compounded as the actualdilution ratio can vary as a function of time.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an example diagram of a system implementing an embodimentof a passive aerosol diluter mechanism;

FIGS. 2A-2D show an embodiment of a dilution-flow orifice fitting formetering airflow;

FIGS. 3A-3D shows an embodiment of a dilution-flow orifice andmixing-tee to dilute a sample particle-laden airflow with clean air(e.g., a substantially particle-free airflow);

FIG. 4A shows an embodiment of a secondary diluter-assemblyincorporating the dilution-flow orifice fitting of FIGS. 2A-2D;

FIG. 4B shows an embodiment of a secondary diluter-assemblyincorporating the dilution-flow orifice and mixing-tee of FIGS. 3A-3D;and

FIG. 5 is a simplified block diagram of a machine in an exemplary formof a computing system within which a set of instructions, for causingthe machine to perform any one or more of the methodologies discussedherein, may be executed.

DETAILED DESCRIPTION

An aerosol diluter mechanism reduces particle concentrations in highparticle-concentration aerosol streams to provide engineers, scientists,and other researchers with a representative sample that meets therecommended operational requirements for CPCs, high-resolutiontime-of-flight spectrometers, and other particle measurement and sizinginstrumentation. The disclosed subject matter is the first to apply flowmonitoring, flow averaging, and dilution ratio monitoring together withmodular sensors to verify function and working state of a passivedilutor that is used with various ones of the particle measurement andsizing instruments discussed herein.

In various embodiments, and with concurrent reference to the figurescontained herein, the disclosed subject matter comprises a passivedilutor consisting of, for example, a filter in parallel with aflow-monitoring device or flow-restricting device, such as an orifice,capillary, or valve. The flow-monitoring device or flow-restrictingdevice (e.g., a ruby orifice or other flow-restricting device known inthe art) allows for the passage of aerosol particles, potentially with amixing orifice, mixing cone, or mixing chamber, or not, to followdownstream of the flow-monitoring device or flow-restricting device.Flow (e.g., determined either as a volumetric flowrate or a massflowrate) through the particle-passing flow-restricting device ismonitored by a pressure and a temperature sensor, such that an accuratedetermination of a flowrate of an aerosol stream through the device canbe measured, based on combining the measured or predetermined flowrate(the nominal flowrate) with thermodynamic properties of one or moregases comprising the aerosol stream, for a given temperature andpressure, to determine a true or actual flowrate. Combined with ameasurement of the total flow through the device, the dilution ratio ofthe dilutor assembly can be constantly or periodically determined insubstantially real-time. Running averages or other such smoothingalgorithms may be applied to the dilution ratio data stream that resultsfrom the passive diluter mechanism. Used in conjunction with an aerosoldetection device, such as a condensation particle counter (CPC), opticalparticle counter (OPC), spectrometer, or other type of particlemonitoring device known in the art (including virtual impactors, cascadeimpactors, and so on), with or without additional dilution, or acatalytic stripper to remove volatile particles (e.g., from emissionsfrom an exhaust from an engine), the dilution ratio from the passivedilutor can be used to correct the detected particle concentration andmeasure the true concentration of particles at the inlet of the dilutermechanism. The determination of the true concentration of particlesallows, among other things, for the total instrument to measure a higherconcentration of aerosol, for instance, from the exhaust of an engine,than the system would be able to measure without dilution.

For example, with reference now to FIG. 1, an example diagram of asystem 100 implementing an embodiment of a passive aerosol dilutermechanism is shown. The system 100 is shown to include a recirculatingprimary dilution portion 110, a primary dilution-flow orifice portion120 for metering airflow, a secondary dilution-flow orifice and mixingportion 130, and a particle measurement portion 140.

The recirculating primary dilution portion 110 includes a sample inletport 101, which accepts a sampled aerosol stream, a primary diluterdevice 103, and a cyclone separator 105. A portion of the sampledaerosol stream continues through a tee 127 onto a catalytic stripper129, while another portion of the aerosol stream continues to anotherbranch of the tee 127, toward a dilution airflow branch. Theconcentrated aerosol stream divides into two paths. In one path, themajority of the original aerosol stream is cleansed of virtually allparticles. In the other path, the remaining small fraction of theaerosol stream retains its original particle concentration. These twopaths then re-combine to produce a pre-determined dilution ratio of theaerosol stream.

The dilution airflow branch includes a filter 113 and a pump 115 to drawthe dilution airflow from the filter 113 and into a desiccant dryer 117.Another filter 113 is located on the downstream side of the desiccantdryer 117 to remove any particles shed from the desiccant materialitself located within the desiccant dryer 117. The dried and filteredairflow then enters the primary dilution-flow orifice portion 120 formetering the airflow. In this example, the primary dilution-flow orificeportion 120 includes a critical orifice 119. The critical orifice 119 isformed from a small hole in a disc (e.g., a ruby disc or plate with asmall hole formed therein), placed transverse to the airflow, and is awell-known means of controlling volumetric airflow at a relativelyconstant rate for a given temperature and pressure. The rate is constantfor a given upstream pressure and temperature of gas in the airflow.Upon reading and understanding the disclosure provided herein, theskilled artisan will recognize that another type of flow-monitoringdevice or flow-restricting orifice, and not necessarily a criticalorifice, may be used along with or as an alternative to the criticalorifice. However, fore as of understanding of the inventive subjectmatter, the examples provided include the critical orifice 119. Thecritical orifice 119 includes a differential-pressure gauge 121, atemperature gauge 123, and an absolute pressure gauge 125. As usedherein, each of the temperature and pressure gauges may be electronictemperature and pressure sensors having at least one of an analog outputand a digital output.

One example of a critical orifice machined or otherwise formed to haveappropriate differential pressure taps, absolute pressure taps, andtemperature taps that may be used within the primary dilution-floworifice portion 120 is disclosed below with regard to FIGS. 2A-2D.Therefore, as discussed in more detail below, the upstream pressure (asmeasured by the absolute pressure of the absolute pressure gauge 125)and the temperature of the airflow (as measured by the temperature gauge123) can be used to determine an actual volumetric airflow in therecirculating primary dilution portion 110 of the system 100. Since theactual volumetric flowrate through the critical orifice 119 is afunction of upstream pressure and temperature of the airflow, both thetemperature and pressure must be monitored. For example, an accuratedilution determination of the passive diluter disclosed herein canchange over time due to, for example, filter loading issues. Thegoverning thermodynamic equations to determine the actual volumetricairflow, once the upstream pressure and temperature of the airflow aredetermined, are known in the art. For example, a hole in the disc of1.07 mm (approximately 0.042 inches) provides an airflow ofapproximately 10.7 lpm at standard temperature and pressure, providedthere is sufficient pressure upstream of the critical orifice. For thisparticular embodiment, the dried and filtered portion of the airflowwithin the recirculating primary dilution portion 110 is about 6.3 lpm.Upon reading and understanding the disclosure provided herein, a skilledartisan would understand how to create other dilution ratios for aparticular application, sampled aerosol stream, and required dilutionneeded for a particular particle monitoring instrument, as described inmore detail below. The differential-pressure gauge 121 provides anotification should the critical orifice 119 become clogged or otherwisemalfunction.

Once the airflow exits the primary dilution-flow orifice portion 120 ofthe system 100, the airflow enters a humidity sensor body 107 thatincludes a temperature gauge 109 and a relative humidity gauge 111 tomonitor the temperature and relative humidity (RH) of the airflowpassing through the humidity sensor body 107. The airflow downstream ofthe humidity sensor body 107 then recombines with the sampled aerosolstream in the primary diluter device 103. A mixing cone, not shown butreadily understood, within the primary diluter device 103, ensures auniform distribution in the diluted aerosol stream.

In an alternative embodiment, not shown explicitly but readilyunderstandable to a skilled artisan, rather than branching off at thetee 127 to the drying and filtering portion of the recirculating primarydilution portion 110, the sampled aerosol stream may simply continuedirectly into the catalytic stripper 129. In this embodiment, a separateairflow (not shown) is provided directly into the filter 113 (upstreamof the pump 115). Such a separate airflow can be provided by a clean,dry air (CDA) system that can be injected directly into the primarydilution-flow orifice portion 120. However, an advantage to utilizingthe tee 127 to split the sampled aerosol stream within the recirculatingprimary dilution portion 110 is that all mass flows within the system100 are balanced (e.g., a sampled aerosol stream sampled at the sampleinlet port 101 will be the same as an outlet airflow stream at anexhaust at an output 157 from the particle measurement portion 140).

With regard to various other elements within the recirculating primarydilution portion 110, the cyclone separator 105 helps removes excesswater vapor and large particles from the sampled aerosol stream.Suitable versions of the cyclone separator 105 are known in the art andare available from a number of suppliers including Parker Hannifin (6035Parkland Boulevard Cleveland, Ohio, USA) and TSI, Inc. (500 CardiganRoad, Shoreview, Minn., USA).

The filter 113 may be a High-Efficiency Particulate Air (HEPA) filter,an Ultra-Low Penetration Air (ULPA) filter, or other type of “absolutefilter” known independently in the art. Suitable versions of the filter113 are available a number of suppliers including from Pall Corporation(25 Harbor Park Drive, Port Washington, N.Y., USA). Each of the filtersin the system 100 identified as the filter 113, may be identical orsimilar to one another provided they are absolute filters.

The pump 115 may be any type of gas-transport pump known in the art.Such pumps include positive-displacement pumps such as rotary vanepumps, diaphragm pumps, and peristaltic pumps depending upon a givenapplication.

The desiccant dryer 117 is used to reduce the atmospheric dew point ofany gas stream, including the dilution airflow, and is known in the art.The desiccant dryer 117 removes additional water vapor from the airflowthat was not trapped by the cyclone separator 105. Suitable versions ofthe desiccant dryer 117 are available from a number of suppliersincluding Parker Hannifin (6035 Parkland Boulevard Cleveland, Ohio, USA)and TSI, Inc. (500 Cardigan Road, Shoreview, Minn., USA).

With continuing reference to FIG. 1, downstream of the tee 127, thecatalytic stripper 129 includes a temperature gauge 131 to measure atemperature of the now-diluted aerosol stream inside the catalyticstripper 129. The catalytic stripper 129 is typically a heated catalyticelement used to remove particle and gas-phase semi-volatile fractions ofthe sampled aerosol stream. The catalytic stripper 129 can be used whenthe sampled aerosol stream includes, for example, particles from dieseland internal-combustion generated exhausts. Therefore, depending on theapplication, the catalytic stripper 129 may not be needed for certaintypes of particle measurement and concentration studies.

An outlet tube 108 downstream of the catalytic stripper 129 transportsthe diluted aerosol stream to the secondary dilution-flow orifice andmixing portion 130 of the system 100, and is pneumatically coupled to atee 118. At the tee 118, the diluted aerosol stream is split where aportion of the diluted aerosol stream is directed into an inlet 116 ofanother critical orifice 119. The remaining portion of the dilutedaerosol stream is directed to the inlet 112 of another filter 113. At anoutlet path 114 of the filter 113, a filtered airstream (clean air)recombines with the diluted aerosol stream, creating a more dilutedaerosol stream.

The critical orifice 119 of the secondary dilution-flow orifice andmixing portion 130 may be the same as or similar to the critical orifice119 of the of the primary dilution-flow orifice portion 120, dependingon a desired secondary dilution ratio. As will be recognizable to theskilled artisan upon reading and understanding the disclosure providedherein, an increased percentage of the aerosol stream may be diverted tothe filter 113 (at either stage of the dilution) and a smallerpercentage of the flow of the aerosol stream is diverted to the criticalorifice 119 (again, at either stage of the dilution). A pre-determinedvolumetric flowrate through the critical orifice 119 determines thedilution ration—a smaller volumetric flowrate through the criticalorifice 119 means that an increased percentage of the aerosol stream isdiverted to the filter 113, creating a higher flowrate of filtered air,thereby creating a higher dilution ratio or the aerosol stream after thetwo streams are recombined.

One example of a critical orifice machined or otherwise formed to haveappropriate differential pressure taps, absolute pressure taps,temperature taps, and including an internal mixing chamber that may beused within the secondary dilution-flow orifice and mixing portion 130is disclosed below with regard to FIG. 3.

Continuing with the example of FIG. 1, an outlet from the secondarydilution-flow orifice and mixing portion 130 is directed into threeseparate flow paths—a sampled flow path 137 that is directed into aparticle monitoring device 141, a bypass flow path 133 that may be usedto provide, for example, a sheath flow for operations of the particlemonitoring device 141, and an excess flow path 139 that joins aerosolstreams from the sampled flow path 137 and the bypass flow path 133 atan outlet (e.g., downstream portion) of the particle monitoring device141. The excess flow path 139 provides a path for a flowrate of theaerosol stream that exceeds the flowrates need for the sampled flow path137 and the bypass flow path 133. For example, frequently, the particlemonitoring device 141 uses a critical orifice 143 in a sampled flow pathinternal to the device to regulate internal volumetric flowrates. In aspecific exemplary embodiment, the volumetric flowrate of the internalsample flow path is 0.1 lpm for a CPC or CNC. The bypass volumetricflowrate may be about 0.6 lpm, and a volumetric flowrate in the excessflow path 139 will be approximately equal to a total volumetric flowrateof the sampled aerosol stream at sample inlet port 101, minus thecombined volumetric flowrate within the sampled flow path 137 and thebypass flow path 133 (in this example, the combined volumetric flowratesare about 0.7 lpm).

In this embodiment, the bypass flow path 133 is coupled to a throttlingdevice 135 upstream of the particle monitoring device 141. Since theaerosol stream through the bypass flow path 133 does not need to beclosely monitored for an actual volumetric flowrate, the throttlingdevice 135 may be, for example, a throttling valve (e.g., a ball valve,gate valve, butterfly valve, or other control valve), a capillaryflow-restricting device, or a critical or non-critical orifice.

In some embodiments of the system 100, the combined aerosol streams fromthe sampled flow path 137, the sampled flow path 137, and the excessflow path 139, at the outlet of the particle monitoring device 141, arethen filtered by means of a filter 113, a pump 115, and another filter113, downstream of the pump 115. Each of the filters 113 and the pump115 may the same as or similar to the other filters and pumps describedherein.

In other examples, the filter 113 upstream of the pump 115, the pump115, and the filter 113 downstream of the pump 115 may not be needed ifthe combined aerosol streams can safely be exhausted to the environment(or into separate filtration system such as a house exhaust or scrubber(not shown)). In other examples, the pump 115 may be used, with orwithout filters, to draw the sampled aerosol stream through the particlemonitoring device 141.

The particle monitoring device 141 may be any of a variety of particlemeasurement devices to measure particle concentrations, particle sizes,particle masses, particle size ranges, and so on depending on the typeof particle study under consideration. For example, the particlemonitoring device 141 may be a CPC, a CNC, an OPC, a particlespectrometer, or other type of particle measurement device known in theart. The particle monitoring device 141 may also include a differentialpressure gauge 151, a temperature gauge 153, and an absolute pressuregauge 155. If the particle monitoring device 141 is a CPC or a CNC, theparticle monitoring device 141 may also include an optics temperaturegauge 145, a condenser temperature gauge 147, and a saturatortemperature gauge 149. In various embodiments, the particle monitoringdevice 141 may include more than one type of measurement device,configured in tandem.

Each of the measured temperatures, pressures, differential pressures,and, in the case of a CPC or CNC, the optics temperature, the condensertemperature, and the saturator temperature, are input into a computingdevice (e.g., a laptop computer, a tablet device, or directly into, forexample, a processor located within the particle monitoring device 141.These variables are then used to determine, in substantially real time,an actual volumetric flowrate in different portions of the system 100.The actual volumetric flowrate is then used to determine an actualdilution ratio of the sampled aerosol stream as discussed in more detailbelow with regard to the governing algorithms.

In various embodiments, the system 100 of FIG. 1 may be incorporated, inwhole or in part, into a particle monitoring device (e.g., a CPC, anOPC, or a spectrometer), or portions of the system 100 may be astandalone passive particle diluter. For example, the secondarydilution-flow orifice and mixing portion 130 may be used or incorporatedwithin an existing particle monitoring device. Also, the number ofdilution stages (e.g., the primary dilution-flow orifice portion 120 andthe secondary dilution-flow orifice and mixing portion 130) may bereduced to a single stage. In other examples, the number of dilutionstages may be increased to increase an overall dilution ratio of asample aerosol stream (e.g., several diluters may be pneumaticallycoupled in series). Further, the skilled artisan will recognize thatdifferent flow rates may be chosen to change dilution ratios to dilute asampled aerosol stream to account for a given particle monitoringdevice, detection efficiencies, response times, and other parameters ofthe particle monitoring device. Such combinations are within the scopeof the inventive subject matter disclosed herein.

Governing Algorithms

As indicated above, the algorithms below are one example of how themeasured temperatures, pressures, and differential pressures are used todetermine, in substantially real time, an actual volumetric flowrate indifferent portions of the system 100 based on thermodynamic propertiesof gases in the aerosol stream and filtered airstream. The actualvolumetric flowrate is then used to determine an actual dilution ratioof the sampled aerosol stream.

With reference to the firmware and software components of the aerosoldiluter mechanism, discussed in more detail below, the algorithms belowinclude relevant portions that, in one embodiment, define how thedilution flow and dilution ratio of the secondary dilutor are used inthe disclosed inventive subject matter. Certain portions of the codeinclude explanations for the related portion. For example, the averagingportion of the code describes how the averaging function operates. Forthis example of the averaging, the first function takes raw numbers fromthe temperature and pressure gauges and converts them into actualmeasurements. The data stream is recorded and feeds forward an averagedversion, an average being based, in this example, on the last fiveseconds worth of data. However, a person of ordinary skill in the art,based on reading and understanding the inventive subject matterdisclosed herein, will understand each portion of the code and,accordingly, will understand how to modify the code for variousoperating and monitoring environments.

This function reads an analog to digital converter associated withvarious ones of the temperature and pressure measurement devices andconvert measured units into appropriate units of kiloPascals andCelsius.

void PNEA::ConvertToUnits( ) {   // Convert all the ADC readings toactuall measurement values   // ADC Main0 - No input on channels 5 and 7  m_PCBTemperature =CalculateTemperature_LM35D(m_ADC_Main0_readings[0]);  m_CondenserTemperature =CalculateThermistor_T(m_ADC_Main0_Voltages[1]);   m_SaturatorTemperature= CalculateThermistor_T(m_ADC_Main0_Voltages[2]);  m_MainTestVoltage5VA = m_ADC_Main0_Voltages[4] * 2.5f;  m_OpticsHeaterVoltage = m_ADC_Main0_Voltages[6] * 2.6f;   // ADCMain1 - No input on channel 7   m_SheathPumpVoltage =m_ADC_Main1_Voltages[0] * 1.2f;   m_DilutionPumpVoltage =m_ADC_Main1_Voltages[1] * 4.0f;   m_CPCPumpVoltage  =m_ADC_Main1_Voltages[2] * 1.357f;   m_MainTestVoltage24V =m_ADC_Main1_Voltages[3] * 12.0f;   m_MainTestVoltage12V =m_ADC_Main1_Voltages[4] * 5.99f;   m_MainTestVoltage3_3V =m_ADC_Main1_Voltages[5] * 1.649f;   m_MainTestVoltage6V =m_ADC_Main1_Voltages[6] * 4.0f;   m_OpticsTemperature  =CalculateThermistor_T(m_ADC_Main0_Voltages[3]);   m_CPCDiffPressure  =CalculateDeltaPressure_MPXV5004(m_ADC_SNSR1_Voltages[7]);  m_CPCAbsPressure  =CalculateAbsolutePressure_MPXAZ6130(m_ADC_SNSR2_Voltages[7]);  m_CPCFlowTemperature = m_OpticsTemperature;   m_BypassAbsPressure =m_CPCAbsPressure;   m_BypassDiffPressure = m_CPCDiffPressure;  m_BypassTemperature = m_CPCFlowTemperature;   m_DilutionAbsPressure =CalculateAbsolutePressure_MPXAZ6130(m_ADC_SNSR2_Voltages[4]);  m_DilutionDiffPressure =CalculateDeltaPressure_MPXV5010(m_ADC_SNSR2_Voltages[1]);  m_DilutionTemperature =CalculateThermistor_T(m_ADC_SNSR2_Voltages[0]);   m_DilutionAbsPressure2= CalculateAbsolutePressure_MPXAZ6130(m_ADC_SNSR0_Voltages[1]);  m_DilutionDiffPressure2 =CalculateDeltaPressure_HSCDRRN002NDAA(m_ADC_SNSR0_Voltages[0]);  m_DilutionTemperature2 =CalculateThermistor_T(m_ADC_SNSR0_Voltages[2]);   m_DryerHumidity =CalculateHumidity_HIH4031(m_ADC_SNSR1_Voltages[0],m_DryerTemperature_Avg.Average( ));   m_DryerTemperature  =CalculateThermistor_T(m_ADC_SNSR1_Voltages[2]);   m_AmbientPressure  =CalculateAbsolutePressure_MPXAZ6130(m_ADC_SNSR1_Voltages[1]);  m_HeatedInletTemperature =CalculateThermistor_T(m_ADC_SNSR1_Voltages[6]); }This function averages the readings.

void PNEA::AverageAll( ) {   m_LaserCurrent_Avg.Add(m_LaserCurrent);  m_BackgroundVoltage_Avg.Add(m_BackgroundVoltage);  m_PCBTemperature_Avg.Add(m_PCBTemperature);  m_CondenserTemperature_Avg.Add(m_CondenserTemperature);  m_SaturatorTemperature_Avg.Add(m_SaturatorTemperature);  m_OpticsHeaterVoltage_Avg.Add(m_OpticsHeaterVoltage);  m_MainTestVoltage5VA_Avg.Add(m_MainTestVoltage5VA);  m_SheathPumpVoltage_Avg.Add(m_SheathPumpVoltage);  m_DilutionPumpVoltage_Avg.Add(m_DilutionPumpVoltage);  m_CPCPumpVoltage_Avg.Add(m_CPCPumpVoltage);  m_MainTestVoltage24V_Avg.Add(m_MainTestVoltage24V);  m_MainTestVoltage12V_Avg.Add(m_MainTestVoltage12V);  m_MainTestVoltage3_3V_Avg.Add(m_MainTestVoltage3_3V);  m_MainTestVoltage6V_Avg.Add(m_MainTestVoltage6V);  m_CPCDiffPressure_Avg.Add(m_CPCDiffPressure);  m_HeatedInletTemperature_Avg.Add(m_HeatedInletTemperature);  m_OpticsTemperature_Avg.Add(m_OpticsTemperature);  m_CPCAbsPressure_Avg.Add(m_CPCAbsPressure);  m_CPCFlowTemperature_Avg.Add(m_CPCFlowTemperature);  m_DilutionAbsPressure_Avg.Add(m_DilutionAbsPressure);  m_DilutionTemperature_Avg.Add(m_DilutionTemperature);  m_DilutionDiffPressure_Avg.Add(m_DilutionDiffPressure);  m_DilutionAbsPressure2_Avg.Add(m_DilutionAbsPressure2);  m_DilutionTemperature2_Avg.Add(m_DilutionTemperature2);  m_DilutionDiffPressure2_Avg.Add(m_DilutionDiffPressure2);  m_BypassAbsPressure_Avg.Add(m_BypassAbsPressure);  m_BypassTemperature_Avg.Add(m_BypassTemperature);  m_BypassDiffPressure_Avg.Add(m_BypassDiffPressure);  m_DryerTemperature_Avg.Add(m_DryerTemperature);  m_AmbientPressure_Avg.Add(m_AmbientPressure);  m_DryerHumidity_Avg.Add(m_DryerHumidity); }This function calculates the secondary dilution ratio.

float32 PNEA::CalculateDilutionRatio2( ) {  float32 DilutionRatio,Temp1, DilutionFlow, BypassFlow;  float32 DilutionTempAvg, CPCTempAvg,BypassTempAvg;  float32 DilutionAbsPressureAvg, CPCAbsPressureAvg, BypassAbsPressureAvg;  float32 DilutionDiffPressureAvg; DilutionTempAvg = m_DilutionTemperature2_Avg.Average( ) +  273.15f; DilutionAbsPressureAvg = m_DilutionAbsPressure2_Avg.Average( ); DilutionDiffPressureAvg = m_DilutionDiffPressure2_Avg.Average( ); CPCTempAvg = m_CPCFlowTemperature_Avg.Average( ) + 273.15f; CPCAbsPressureAvg = m_CPCAbsPressure_Avg.Average( );  BypassTempAvg =m_BypassTemperature_Avg.Average( ) + 273.15f;  BypassAbsPressureAvg =m_BypassAbsPressure_Avg.Average( );  if (m_bDilutionRatioEn){  DilutionFlow =   m_DilutionFlow2.m_StandardFlowRateAvg.Average( );   //Assuming Bypass and CPC Flow are at same Temperature    andPressure   BypassFlow =   m_BypassFlow.m_StandardFlowRateAvg.Average( );  Temp1 = m_CPCFlow.m_StandardFlowRateAvg.Average( ) +   BypassFlow;   if (DilutionFlow == 0)     DilutionRatio = 0.0f;    else    {    DilutionRatio = Temp1 / DilutionFlow;    }  }  else   DilutionRatio= 1.0;  return DilutionRatio; }

A skilled artisan will recognize that other algorithms, based onprinciples of thermodynamics, may be used with the system 100 of FIG. 1.The algorithms above are provided to illustrate more fully how thevarious temperature, pressure, differential pressures, and so on areused to determine actual dilution ratios. For example, the skilledartisan will recognize how to apply the appropriate algorithms if moredilution stages are added.

Referring now to FIGS. 2A-2D, an embodiment of a dilution-flow orificefitting 200 for metering airflow is shown. For example, FIG. 2A is a topview of the dilution-flow orifice fitting 200 and is shown to include anaerosol stream inlet port 201, an aerosol stream outlet port 203, and apair of differential pressure ports 205 all machined or otherwise formedas part of the dilution-flow orifice fitting 200. The pair ofdifferential pressure ports 205 is mounted on either side (upstream anddownstream sides) of an internal critical orifice, discussed below withreference to FIG. 2D. As noted above, the dilution-flow orifice fitting200 is used to meter either a filtered (clean) airflow or aparticle-laden airflow (e.g., aerosol stream). In a specific exemplaryembodiment, a dimension D₁ from the centerlines of the differentialpressure ports 205 is about 11.4 mm (approximately 0.450 inches). Thedilution-flow orifice fitting 200 may be machined or otherwise formedfrom a variety of materials, as discussed in more detail below.

FIG. 2B shows a front (elevation) view of the dilution-flow orificefitting 200. In a specific exemplary embodiment, a dimension D₂,indicating a length of a main portion of the dilution-flow orificefitting 200, is about 15.2 mm (approximately 0.600 inches), a dimensionD₃, indicating an overall length of the dilution-flow orifice fitting200, is about 30.5 mm (approximately 1.20 inches), and a dimension D₄,indicating an overall height of the dilution-flow orifice fitting 200,is about 16.5 mm (approximately 0.650 inches).

FIG. 2C shows a side (elevation) view, looking from the aerosol streaminlet port 201. FIG. 2C also indicates a cross-section, labeled 2D-2D,that is discussed with reference to FIG. 2D, below. In a specificexemplary embodiment, a dimension D₅, indicating an overall width of thedilution-flow orifice fitting 200, is about 11.4 mm (approximately 0.450inches).

FIG. 2D shows a front (elevation) cross-sectional view. Thedilution-flow orifice fitting 200 is shown to include a critical orifice207. The critical orifice 207 may the same as or similar to the criticalorifice 119 discussed above with reference to FIG. 1. Notice that thecritical orifice 207 is located, from a fluid mechanics perspective,between the differential pressure ports 205 to allow monitoring of thedifferential pressure across the critical orifice 207. In a specificexemplary embodiment, a dimension D₆, indicating a diameter of theaerosol stream inlet port 301, is about 3.81 mm (approximately 0.150inches).

With regard to FIGS. 2A-2D overall, in a specific exemplary embodiment,the dilution-flow orifice fitting 200 is machined or otherwise formedfrom stainless steel (e.g., 316L stainless). However, the skilledartisan will recognize that other suitable materials may be used aswell, provided that electrostatic attraction does not remove asignificant percentage of particles from the airflow. For example,friction induced by airflow through a plastic version of thedilution-flow orifice fitting 200 can induce a static charge on plastic(unless coated with, for example, a electrically-conductive material). Asignificant number of particles in the airflow, especially thoseparticles less than a few microns in diameter, will be removed byelectrostatic attraction of the particles to the plastic body of thedilution-flow orifice fitting 200. Therefore, materials from which thedilution-flow orifice fitting 200 is formed need to be considered. Theskilled artisan will further recognize that each of the dimensionsprovided above are merely examples and are given only to illuminate morefully various exemplary embodiments that may be used to produce thedilution-flow orifice fitting 200. For example, the various dimensionsshown may be increased to allow for much higher volumetric flowrates.Alternatively, the various dimensions shown may be decreased to allowfor much lower volumetric flowrates, while concurrently maintaining asmall form factor to fit within, for example, a particle monitoringdevice (e.g., such as the particle monitoring device 141 of FIG. 1).

Referring now to FIGS. 3A-3D, an embodiment of a dilution-flow orificeand mixing-tee 300 to dilute a sample particle-laden airflow with cleanair (e.g., a substantially particle-free airflow) is shown. For example,FIG. 3A is a top view of the dilution-flow orifice and mixing-tee 300and is shown to include an aerosol stream inlet port 301, a clean airinlet port 303, and a pair of differential pressure ports 305 allmachined or otherwise formed as part of the dilution-flow orifice andmixing-tee 300. The pair of differential pressure ports 305 is mountedon either side (upstream and downstream sides) of an internal criticalorifice, discussed below with reference to FIG. 3D. As noted above, thedilution-flow orifice and mixing-tee 300 is used to both meter either aclean airflow or a particle-laden airflow (e.g., aerosol) and mix theaerosol stream and filtered airstream together, thereby producing adiluted aerosol stream at the clean air inlet port 303. In a specificexemplary embodiment, a dimension D₇ from the centerlines of thedifferential pressure ports 305 is about 11.4 mm (approximately 0.450inches). The dilution-flow orifice and mixing-tee 300 may be machined orotherwise formed from a variety of materials, as discussed in moredetail below.

FIG. 3B shows a front (elevation) view of the dilution-flow orifice andmixing-tee 300. In a specific exemplary embodiment, a dimension D₈,indicating a length of a main portion of the dilution-flow orifice andmixing-tee 300, is about 15.2 mm (approximately 0.600 inches), adimension Dg, indicating an overall length of the dilution-flow orificeand mixing-tee 300, is about 30.5 mm (approximately 1.20 inches), adimension D₁₀, indicating an overall height of the dilution-flow orificeand mixing-tee 300, is about 24.1 mm (approximately 0.950 inches), and adimension D₁₁, indicating a height of the main portion of the body ofthe dilution-flow orifice and mixing-tee 300 to an uppermost portion ofthe pair of differential pressure ports, is about 16.5 mm (approximately0.650 inches).

FIG. 3C shows a side (elevation) view, looking from the aerosol streaminlet port 301. FIG. 3C also indicates a cross-section, labeled 3D-3D,that is discussed with reference to FIG. 3D, below. In a specificexemplary embodiment, a dimension D₁₂, indicating an overall width ofthe dilution-flow orifice and mixing-tee 300, is about 11.4 mm(approximately 0.450 inches).

FIG. 3D shows a front (elevation) cross-sectional view. Thedilution-flow orifice and mixing-tee 300 is shown to include a criticalorifice 309. The critical orifice 309 may the same as or similar to thecritical orifice 119 discussed above with reference to FIG. 1. Noticethat the critical orifice 309 is located, from a fluid mechanicsperspective, between the differential pressure ports 305 to allowmonitoring of the differential pressure across the critical orifice 309.In a specific exemplary embodiment, a dimension D₁₃, indicating adiameter of the aerosol stream inlet port 301, is about 3.81 mm(approximately 0.150 inches).

With regard to FIGS. 3A-3D overall, in a specific exemplary embodiment,the dilution-flow orifice and mixing-tee 300 is machined or otherwiseformed from stainless steel (e.g., 316L stainless). For example, thedilution-flow orifice and mixing-tee 300 may be formed from the same ora similar material used to form the dilution-flow orifice fitting 200 ofFIGS. 2A-2D. However, the skilled artisan will recognize that othersuitable materials may be used as well, provided that electrostaticattraction does not remove a significant percentage of particles fromthe airflow. For example, as noted above with reference to thedilution-flow orifice fitting 200, friction induced by airflow through aplastic version of the dilution-flow orifice and mixing-tee 300 caninduce a static charge on plastic. A significant number of particles inthe airflow, especially those particles less than a few microns indiameter, will be removed by electrostatic attraction of the particlesto the plastic body of the dilution-flow orifice and mixing-tee 300.Therefore, materials from which the dilution-flow orifice and mixing-tee300 is formed need to be considered. The skilled artisan will furtherrecognize, as with the dilution-flow orifice fitting 200 discussedabove, that each of the dimensions provided above are merely examplesand are given only to illuminate more fully various exemplaryembodiments that may be used to produce the dilution-flow orifice andmixing-tee 300.

Upon reading and understanding the disclosure provided herein, theskilled artisan will further recognize that the inventive subject mattercan be practiced without the actual fittings of FIGS. 2A-2D and FIGS.3A-3D. The fittings in these figures represent one way in which toimplement embodiments of the disclosed subject matter. However, theinventive subject matter of FIG. 1 can be practiced with “discretecomponents” as indicated in FIG. 1. That is, critical orifices,temperature and pressure gauges, and filters can be used as indicated inFIG. 1 to practice embodiments of the invention.

For example, FIG. 4A shows an embodiment of a secondary diluter-assemblyincorporating the dilution-flow orifice fitting of FIGS. 2A-2D. However,rather than using the dilution-flow orifice and mixing-tee 300 of FIGS.3A-3D, discrete components are used to constitute a mixing portion(combining aerosol streams with a filtered airstream) to produce adiluted aerosol stream.

With concurrent reference to FIG. 1, FIG. 4A is shown to include thecatalytic stripper 129, an outlet tube 108 from the catalytic stripper129, the tee 118, the inlet 116 to the dilution-flow orifice fitting200, the differential pressure ports 205 from the dilution-flow orificefitting 200, the filter 113, a secondary tee 401 downstream of thefilter 113, and the critical orifice 119. To avoid obscuring theembodiment of FIG. 4A, the temperature and pressure gauges within thesecondary dilution-flow orifice and mixing portion 130 of FIG. 1 are notshown in FIG. 4A. In the embodiment of FIG. 4A, the critical orifice 119of FIG. 1 is contained within the dilution-flow orifice fitting 200 ofFIG. 4A. The “mixing function” (of the aerosol stream and a filteredairstream) of the dilution-flow orifice and mixing-tee 300 of FIGS.3A-3D is not used in FIG. 4A. Instead, the mixing function is performedby a combination of the tee 118, the filter 113 and the dilution-floworifice fitting 200, coupled with the aerosol stream, downstream fromthe dilution-flow orifice fitting 200, and the filtered airstream,downstream of the filter 113, being rejoined and combined (e.g., mixed)in the secondary tee 401, as will be recognizable in view of thesecondary dilution-flow orifice and mixing portion 130 of FIG. 1. (Thecritical orifice 119 of FIG. 4A, downstream of the filter 113, isoptional and may be used for a low-flow pressure monitoring device andis not shown in FIG. 1.)

FIG. 4B shows an embodiment of a secondary diluter-assemblyincorporating the dilution-flow orifice and mixing-tee of FIGS. 3A-3D.Again, with concurrent reference to FIG. 1, FIG. 4B is shown to includethe catalytic stripper 129, an outlet tube 108 from the catalyticstripper 129, the tee 118, the inlet 116 to the aerosol stream inletport 301 of the dilution-flow orifice and mixing-tee 300, thedifferential pressure ports 305 from the dilution-flow orifice andmixing-tee 300, the filter 113, the outlet path 114 from the filter 113to the clean air inlet port 303 of the dilution-flow orifice andmixing-tee 300, and the sampled flow path 137 that continues to theparticle monitoring device 141 (not shown in FIG. 4B). To avoidobscuring the embodiment of FIG. 4B, the temperature and pressure gaugeswithin the secondary dilution-flow orifice and mixing portion 130 ofFIG. 1 are not shown in FIG. 4B.

Based on reading and understanding the disclosure of the inventivesubject matter provided herein, the skilled artisan can readily envisionother ways to incorporate various combinations of the dilution-floworifice fitting 200 and the dilution-flow orifice and mixing-tee 300,along with various discrete components, that are all within a scope ofthe present disclosure. The skilled artisan will also recognize thatfewer or more dilution stages may be used as needed for a particularapplication.

Therefore, included in the disclosed subject matter are a system diagramdescribing various embodiments of the passive aerosol diluter mechanism.Firmware or software, as discussed above with regard to the variousalgorithms, are used to correct particle concentrations for one or moreactual dilution ratios, and may also be included as a portion of theoverall system. The firmware or software may be incorporated into theparticle monitoring device 141 (e.g., a CPC, CNC, OPC, or spectrometer)used to monitor the diluted particle stream, as a separate part of thesystem, or as a separate standalone component that may be run on varioustypes of computers, laptops, tablets, or other computing devices foreither concurrent or later analysis of the recorded measurements. Thefittings that allow for pressure, temperature, and flow measurements tobe made in the passive aerosol diluter mechanism may also be included.In various embodiments, measurements of differential pressure, absolutepressure, and temperature across various ones of the filters,flow-restriction devices, throttling valves, critical orifices, and soon can be monitored at predetermined intervals (e.g., minute-by-minute,every five seconds, every second, every one-tenth of a second, etc.).Also, various dilution ratios (e.g., 10⁵ to 1, 10³ to 1, 10 to 1, etc.)may be predetermined as needed for a given particle concentration rangeand a known or calculated coincidence-error loss for a given measurementinstrument (e.g., a CPC).

Also, as indicated above, the system can be configured to providemeasurements based on volumetric or mass-based flow and calculations toallow these determinations and may also be included in, for example,firmware or software. The disclosed passive aerosol diluter mechanismalso allows the “health” of the dilutor to be monitored, such thatchanges in dilution ratio, various pressures or temperatures, or flowsmay indicate that the filters or various ones of the flow-restrictingdevices are becoming or are plugged may be configured to generate anerror to notify the end user as to various maintenance issues areneeded. Further, the disclosed passive aerosol diluter mechanism can beconfigured to employ a modular sensor system and unique signalprocessing, including running averages, to perform passive dilution ofan aerosol stream, thereby resulting in a very compact package.Therefore, the disclosed passive aerosol diluter mechanism solves theproblem of having a robust and accurate dilution device to measure highconcentrations of particles in the aerosol stream, such as those emittedfrom internal combustion and diesel engines. Moreover, the passiveaerosol diluter mechanism may be configured as a drop-in replacement fornon-automated types of diluters.

Exemplary Machine Architecture and Machine-Readable Storage Medium

With reference now to FIG. 5, an exemplary embodiment extends to amachine in an example of a computer system 500 within whichinstructions, for causing the machine to perform any one or more of themethodologies discussed herein, may be executed. In alternativeexemplary embodiments, the machine operates as a standalone device ormay be connected (e.g., networked) to other machines. In a networkeddeployment, the machine may operate in the capacity of a server or aclient machine in server-client network environment, or as a peermachine in a peer-to-peer (or distributed) network environment. Themachine may be a personal computer (PC), a tablet PC, a set-top box(STB), a Personal Digital Assistant (PDA), a cellular telephone, a webappliance, a network router, a switch or bridge, or any machine capableof executing instructions (sequential or otherwise) that specify actionsto be taken by that machine. Further, while only a single machine isillustrated, the term “machine” shall also be taken to include anycollection of machines that individually or jointly execute a set (ormultiple sets) of instructions to perform any one or more of themethodologies discussed herein.

The computer system 500 includes a processor 501 (e.g., a centralprocessing unit (CPU), a graphics processing unit (GPU) or both), a mainmemory 503 and a static memory 505, which communicate with each othervia a bus 507. The computer system 500 may further include a videodisplay unit 509 (e.g., a liquid crystal display (LCD) or a cathode raytube (CRT)). The computer system 800 also includes an alphanumeric inputdevice 511 (e.g., a keyboard), a user interface (UI) navigation device513 (e.g., a mouse), a disk drive unit 515, a signal generation device517 (e.g., a speaker), and a network interface device 519.

Machine-Readable Medium

The disk drive unit 515 includes a non-transitory machine-readablemedium 521 on which is stored one or more sets of instructions and datastructures (e.g., software 823) embodying or used by any one or more ofthe methodologies or functions described herein. The software 523 mayalso reside, completely or at least partially, within the main memory503 or within the processor 501 during execution thereof by the computersystem 500; the main memory 803 and the processor 801 also constitutingmachine-readable media.

While the non-transitory machine-readable medium 521 is shown in anexemplary embodiment to be a single medium, the term “machine-readablemedium” may include a single medium or multiple media (e.g., acentralized or distributed database, or associated caches and servers)that store the one or more instructions. The term “non-transitorymachine-readable medium” shall also be taken to include any tangiblemedium that is capable of storing, encoding, or carrying instructionsfor execution by the machine and that cause the machine to perform anyone or more of the methodologies of the present invention, or that iscapable of storing, encoding, or carrying data structures used by orassociated with such instructions. The term “non-transitorymachine-readable medium” shall accordingly be taken to include, but notbe limited to, solid-state memories, and optical and magnetic media.Specific examples of machine-readable media include non-volatile memory,including by way of exemplary semiconductor memory devices (e.g., EPROM,EEPROM, and flash memory devices); magnetic disks such as internal harddisks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROMdisks.

Transmission Medium

The software 523 may further be transmitted or received over acommunications network 525 using a transmission medium via the networkinterface device 519 utilizing any one of a number of well-knowntransfer protocols (e.g., HTTP). Examples of communication networksinclude a local area network (LAN), a wide area network (WAN), theInternet, mobile telephone networks, Plain Old Telephone (POTS)networks, and wireless data networks (e.g., WiFi and WiMax networks).The term “transmission medium” shall be taken to include any intangiblemedium that is capable of storing, encoding, or carrying instructionsfor execution by the machine, and includes digital or analogcommunications signals or other intangible medium to facilitatecommunication of such software.

Although an overview of the inventive subject matter has been describedwith reference to specific exemplary embodiments, various modificationsand changes may be made to these embodiments without departing from thebroader spirit of the present invention. Such embodiments of theinventive subject matter may be referred to herein, individually orcollectively, by the term “invention” merely for convenience and withoutintending to voluntarily limit the scope of this application to anysingle invention or inventive concept if more than one is, in fact,disclosed.

The description above includes illustrative examples, devices, andapparatuses that embody the disclosed subject matter. In thedescription, for purposes of explanation, numerous specific details wereset forth in order to provide an understanding of various embodiments ofthe inventive subject matter. It will be evident, however, to those ofordinary skill in the art that various embodiments of the inventivesubject matter may be practiced without these specific details. Further,well-known structures, materials, and techniques have not been shown indetail, so as not to obscure the various illustrated embodiments.

As used herein, the term “or” may be construed in an inclusive orexclusive sense. Additionally, although various exemplary embodimentsdiscussed herein focus on particular ways to dilute a sampled particlestream, other embodiments will be understood by a person of ordinaryskill in the art upon reading and understanding the disclosure provided.Further, upon reading and understanding the disclosure provided herein,the person of ordinary skill in the art will readily understand thatvarious combinations of the techniques and examples provided herein mayall be applied serially or in various combinations.

Although various embodiments are discussed separately, these separateembodiments are not intended to be considered as independent techniquesor designs. As indicated above, each of the various portions may beinter-related and each may be used separately or in combination withother diluter techniques discussed herein.

Moreover, although specific values, ranges of values, measurementvalues, and techniques are given for various parameters discussedherein, these values and techniques are provided merely to aid theperson of ordinary skill in the art in understanding certaincharacteristics of the designs and techniques disclosed herein. Those ofordinary skill in the art will realize, upon reading and understandingthe disclosure provided, that these values and techniques are presentedas examples only and numerous other values, ranges of values, andtechniques may be employed while still benefiting from the novel designsdiscussed that may be employed to dilute a given particle-laden samplestream. Therefore, the various illustrations of the apparatus areintended to provide a general understanding of the structure and designof various embodiments and are not intended to provide a completedescription of all the elements and features of the apparatus that mightmake use of the structures, features, and designs described herein.

Many modifications and variations can be made, as will be apparent tothe person of ordinary skill in the art upon reading and understandingthe disclosure provided herein. Functionally equivalent methods anddevices within the scope of the disclosure, in addition to thoseenumerated herein, will be apparent to a person of ordinary skill in theart from the foregoing descriptions. Portions and features of someembodiments may be included in, or substituted for, those of others.Many other embodiments will be apparent to those of ordinary skill inthe art upon reading and understanding the description provided. Suchmodifications and variations are intended to fall within a scope of theappended claims. Therefore, the present disclosure is to be limited onlyby the terms of the appended claims, along with the full scope ofequivalents to which such claims are entitled. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only and is not intended to belimiting.

The Abstract of the Disclosure is provided to allow the reader toquickly ascertain the nature of the technical disclosure. The abstractis submitted with the understanding that it will not be used tointerpret or limit the claims. In addition, in the foregoing DetailedDescription, it may be seen that various features may be groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted aslimiting the claims. Thus, the following claims are hereby incorporatedinto the Detailed Description, with each claim standing on its own as aseparate embodiment.

1. A system to dilute a sampled aerosol stream, the system comprising:an aerosol sample inlet to receive a sample of an aerosol stream; a flowdiverter device to split the sampled aerosol stream into a first portionof the sampled aerosol stream and a remaining portion of the sampledaerosol stream; a flow-monitoring device coupled to the flow diverterdevice to receive the first portion of the sampled aerosol stream fromthe aerosol sample inlet, the flow-monitoring device being configured toallow for a passage of particles contained in the sampled aerosolstream; a filter coupled in fluid communication with the flow diverterdevice and the flow-monitoring device to receive the remaining portionof the sampled aerosol stream from the aerosol sample inlet to provide adiluted aerosol stream, the diluted aerosol stream to be coupled to theflow-monitoring device; and a pressure sensor and a temperature sensorto monitor a pressure and a temperature of the aerosol stream upstreamof the flow-monitoring device.
 2. The system of claim 1, wherein thepressure sensor is an absolute pressure sensor configured to monitor apressure of the aerosol stream entering the flow-monitoring device. 3.The system of claim 1, further comprising a mixing chamber coupled to anoutlet of the both the flow-monitoring device and the filter to providethe diluted aerosol stream.
 4. The system of claim 1, wherein theflow-monitoring device is a critical orifice.
 5. The system of claim 4,further comprising sizing an orifice for a predetermined nominalvolumetric flowrate through the critical orifice.
 6. The system of claim1, further comprising a secondary dilution system coupled to an outletof both the flow-monitoring device and the filter.
 7. The system ofclaim 1, further comprising coupling an outlet of both theflow-monitoring device and the filter to a particle monitoring device.8. The system of claim 7, wherein the particle monitoring device is acondensation particle counter.
 9. The system of claim 1, furthercomprising placing a differential pressure sensor across theflow-monitoring device to measure a differential pressure of theflow-monitoring device.
 10. The system of claim 1, further comprising apressure sensor and a temperature sensor to monitor a pressure and atemperature of the aerosol stream upstream of the filter.
 11. The systemof claim 1, further comprising a catalytic stripper in series with theflow-monitoring device and the filter, the catalytic stripper to removevolatile particles from the aerosol stream.
 12. A method of diluting asampled aerosol stream, the method comprising: receiving a sample of anaerosol stream; diverting a first portion of the aerosol stream to aflow-monitoring device; monitoring a temperature and a pressure of theflow-monitoring device; diverting a remaining portion of the aerosolstream to a filter; filtering the remaining portion of the aerosolstream to produce a filtered airstream; and combining the first portionof the aerosol stream from an outlet of the flow-monitoring device withthe filtered airstream.
 13. The method of claim 12, further comprisingpredetermining a nominal flowrate through the flow-monitoring device.14. The method of claim 12, further comprising: calculating an actualflowrate of the filtered airstream; calculating an actual flowrate ofthe first portion of the aerosol stream from the outlet of theflow-monitoring device; and calculating a dilution ratio based on aratio of the actual flowrate of the first portion of the aerosol streamfrom the outlet of the flow-monitoring device divided by the actualflowrate of the filtered airstream.
 15. The method of claim 14, whereinthe calculations of actual flowrates are determined by thermodynamicproperties of one or more gases comprising the aerosol stream based onthe monitored temperature and the monitored pressure of theflow-monitoring device.
 16. The method of claim 14, further comprisingcalculating running averages of the actual flowrates.
 17. The method ofclaim 14, further comprising applying a smoothing function to the actualflowrates.
 18. The method of claim 12, further comprising predetermininga time interval over which to update actual determinations of the actualflowrate of the first portion of the aerosol stream from the outlet ofthe flow-monitoring device.
 19. A computer-readable storage mediumstoring instructions that, when executed by one or more processors,cause the one or more processors to perform operations comprising:receiving a data stream including a temperature and a pressure of anaerosol stream; receiving a predetermined nominal flowrate of theaerosol stream; calculating an actual flowrate of filtered air divertedfrom the aerosol stream to a filter, the actual flowrate of the filteredair being a portion of the flowrate of the aerosol stream; calculatingan actual flowrate of a remaining portion of the aerosol stream divertedfrom the aerosol stream to a flow monitoring device, the actual flowratecalculated as coming from an outlet of the flow-monitoring device; andcalculating a dilution ratio based on a ratio of the actual flowrate ofthe remaining portion of the aerosol stream from the outlet of theflow-monitoring device divided by the actual flowrate of the filteredair.
 20. The computer-readable storage medium of claim 19, wherein thecalculations of actual flowrates are determined by thermodynamicproperties of one or more gases comprising the aerosol stream based onthe monitored temperature and the monitored pressure of theflow-monitoring device.